Dietary fibers are generic term referring to ingredients in foods that cannot be digested with human digestive enzymes. Because dietary fibers have various effects, they have been attracting attentions as “the sixth nutrient” in addition to the five major nutrients. Dietary fibers can roughly be divided into water-soluble dietary fibers (hereinafter referred to as SDF) and water-insoluble dietary fibers (hereinafter referred to as IDF). Since these soluble- and insoluble-dietary fibers exhibit different physiological functions in the body, respective values for both SDF and IDF are indicated in food composition tables. In addition, the total value for both fibers (SDF and IDF) is also displayed in food composition tables as the total content of all dietary fibers.
One example of physiological functions of dietary fibers is that SDF is known to be prone to fermentation in colon, so that the fermented products exhibit many different types of physiological functions. SDF can be divided into fermentable SDF and non-fermentable SDF depending on their fermentability in colon. The former is utilized by intestinal bacteria to generate short chain fatty acids in colon, which would then serve as energy sources for the intestinal tract, and gives chemical stimuli to the intestinal tract, giving rise to control the intestinal conditions such that bowel movement become improved. The latter increases the bowel volume, due to its water retentivity and gives physical stimuli to the intestinal tract, which then lead to improvement of Taxation. On the other hand, there are several reports focusing on IDF and its excretion stimulating effect. Further, it is known that increase of IDF intake suppresses onsets of colorectal cancer and diverticulosis of colon as a result of stimulating movement of the bowel content. Therefore, it is recognized that SDF and IDF are different in physiological functions, and hence, intake of these dietary fibers in a balanced manner appears to be of importance.
In view of such findings and increase of demand for health trend, resistant starch (hereinafter referred to as RS) products, which are starch raw materials with higher contents of dietary fibers, i.e., with increased resistance against digestive enzymes, are on the market from various manufacturers. Methods for providing enzyme-resistance to starch includes: process for cross-linking a starch by phosphate (Patent Document 1); treatment of a starch with heat-moisture (Patent Document 2, non-Patent Documents 1 and 2); and selecting and processing a starch with high amylose content (Patent Documents 3 and 4).
Gelatinization properties are characteristic for starch raw materials used in food, and such properties of these RS are significantly repressed. Therefore, grains of these RS do not swell even by heating in a slurry state. With the characteristics, RS can exhibit resistance against digestive enzymes. However, due to the characteristics, RS-added foods may often leave powdery or rough texture in the mouth. Further, it has been difficult to add viscosity such as thickness to foods using these RS products.
One of commonly used methods for measuring dietary fiber content in RS is AOAC method 985.29 (commonly referred to as Prosky's method). Although Prosky's method can detect IDF and SDF with high-molecular weight which become precipitated by treatment with alcohol (high-molecular weight water-soluble dietary fibers: HSDF), it cannot detect SDF with low-molecular weight which do not precipitate by alcohol treatment (low-molecular weight water-soluble dietary fibers: LSDF) due to the principle of measurement. One of the method which can detect such low-molecular weight SDF (LSDF) is AOAC method 2001.03, a method for quantitating dietary fibers containing indigestible dextrins. Therefore, LSDF content in a given sample can be identified by comparing the quantitative values obtained through these methods.
IDFHSDFLSDFProsky's method∘∘xAOAC method 2001.03∘∘∘∘: counted in the quantitative valuex: not counted in quantitative value
Existing water-soluble dietary fibers includes pectic substances, vegetable gums (such as guar gums), viscous substances (such as mannan), and seaweed polysaccharides (such as alginic acid, laminarin and fucoidan). Addition of a pectic substance, guar gum, mannan or the like to food enhances effects on physical properties of dough and batter during food production, as well as effects on the shape and mouthfeel of the final product, depending on the volume to be added. Furthermore, because water-soluble dietary fibers have significant effect on properties and conditions of dough and batter, there was a difficulty in applying such fibers, without any modification, to food made by a process involving steps for producing dough or batter. When water-soluble and low-viscous dietary fibers are used in a process of making breads and confectioneries, which involve the step of expansion, they prevent the expansion which results in problems such as insufficient cooking of the final product or hardening of the surface thereof.
RS-containing noodle is disclosed as one type of RS-containing food latent Documents 5 and 6). The contents of dietary fibers in RS disclosed in these Documents are defined by Prosky's method, i.e. by the total amount of water-insoluble dietary fibers (IDF) and high-molecular weight water-soluble dietary fibers (HSDF), and the amount of low-molecular weight water-soluble dietary fiber (LSDF) is not mentioned therein.
Meanwhile, hydroxypropyl starch (hereinafter referred to as PO starch) and hydroxypropylated phosphate cross-linked starch (hereinafter referred to as PO cross-linked starch) are also known as RS other than those described above. For example, non-Patent Document 3 describes that PO cross-linked starch is less susceptible to alpha-amylase digestion compared to oxidized starch, acetylated phosphate cross-linked starch and dextrins. Non-Patent Document 4 discloses that PO starch acquires resistance against hydrolysis by porcine pancreatic alpha-amylase, depending on degree of substitution thereon. Further, Patent Document 7 describes that hydrolysates of PO starch have low calorie. Non-Patent Document 5 reports on increases in the amounts of bile acids and cholesterol to be excreted, hypertrophy of and pH reduction in cecum, and alteration of organic acid compositions in rat models fed with food containing either PO starch or PO cross-linked starch. Furthermore, Patent Document 8 discloses a water-soluble dietary fiber having low viscosity and a composition for dietary food enrichment containing PO starches with average degree of substitution between 0.02-0.2.
However, starches, in particular, PO starch and PO cross-linked starch have not yet been used as water-soluble dietary fiber enriching agents.                [Patent Document 1] U.S. Pat. No. 5,855,946 (JP-T-2002-503959)        [Patent Document 2] Japanese Patent No. 3530567        [Patent Document 3] JP-T-08-504583        [Patent Document 4] JP-A-11-5802        [Patent Document 5] Japanese Patent No.3798509        [Patent Document 6] JP-A-2006-129790        [Patent Document 7] JP-B-07-14331        [Patent Document 8] JP-A-10-243777        [Non-Patent Document 1] Mohd et al, Starch/Staerke, 36(8), 273-275, 1984        [Non-Patent Document 2] Starch Science, Vol. 40(3), Pages 285-290, 1993        [Non-Patent Document 3] Journal of Japanese Society of Nutrition and Food Science, Vol. 45(6), Pages. 551-553, 1992        [Non-Patent Document 4] M.Wootton et al, Starch/Staerke, 33(4), 135-137,1981        [Non-Patent Document 5] J. Nutr., 128, 848-854, 1998        